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51.
The non-random dispersal of plant propagules is thought to counter competitive exclusion and thus promote the survival of competitively inferior species. We investigated this process by modelling the outcome of interactions between species with competitive ability defined as a function of both life-history traits and the environment with both random and clustered dispersal strategies and in environmentally homogeneous and heterogeneous environments. Four main results emerged: (1) environmental heterogeneity was seen to promote co-existence in conjunction with associated trait variation for tolerance to the environmental variable and where this trait variation was effectively limited by ‘trade-off’ such that no single species had an overall competitive advantage, (2) consistent with theory, random dispersal appeared to enhance the likelihood of competitive exclusion, whereas clustering favoured co-existence, (3) the ecological outcome of interactions between dispersal and competitive relationships varied as a function of the trait determining the competitive advantage within a particular environment, and (4) promotion of co-existence by clustered dispersal was most marked when associated with environmental heterogeneity. It is argued that these results suggest that current ecological models of species interactions may need to be modified to incorporate a more realistic understanding of competitive ability if we are to better understand the factors effecting species co-existence. 相似文献
52.
Cystinosis was diagnosed in the first trimester of pregnancy by studies of uptake and retention of [35S]-cystine by intact biopsy samples of chorionic villi. The diagnosis was confirmed by similar studies on cell cultures of villi and fetal skin fibroblasts. 相似文献
53.
Gearheard S Matumeak W Angutikjuaq I Maslanik J Huntington HP Leavitt J Kagak DM Tigullaraq G Barry RG 《Ambio》2006,35(4):203-211
The Arctic environment, including sea ice, is changing. The impacts of these changes to Inuit and I?upiat ways of life vary from place to place, yet there are common themes as well. The study reported here involved an exchange of hunters, Elders, and others from Barrow, Alaska, USA, and Clyde River, Nunavut, Canada, as members of a larger research team that also included visiting scientists. Although the physical environments of Barrow and Clyde River are strikingly different, the uses of the marine environment by residents, including sea ice, had many common elements. In both locations, too, extensive changes have been observed in recent years, forcing local residents to respond in a variety of ways. Although generally in agreement or complementary to one another, scientific and indigenous knowledge of sea ice often reflect different perspectives and emphases. Making generalizations about impacts and responses is challenging and should therefore be approached with caution. Technology provides some potential assistance in adapting to changing sea ice, but by itself, it is insufficient and can sometimes have undesirable consequences. Reliable knowledge that can be applied under changing conditions is essential. Collaborative research and firsthand experience are critical to generating such new knowledge. 相似文献
54.
55.
Sergio Rinaldi Warren Sanderson Alessandra Gragnani 《Journal of environmental management》1996,48(4):357-373
Policies for the management of natural resources and the resources themselves interact to form complex systems. In this paper, we present a highly simplified model that can be used to study the general features of those systems. The model has three state variables, the abundance of the resource, environmental pollution, and the capital devoted to pollution control. We analyse it graphically using the singular perturbation approach. Two modes of behavior are possible; stationary and cyclic. When the abundance of the resources varies cyclically, the length of the period of resource scarcity depends on population size, economic activity, pollution per unit of output, and policy constraints. We distinguish between two classes of policies, one in which decision-makers base their investments in pollution control capital on the abundance of the resource, and another in which those decisions depend on the amount of pollution. We show that policies based on the observation of pollution are safer than those based on resource abundance, because in the latter case, small changes in policy variables can lead much more easily to a collapse of the resource. Increases in population size, even when accompanied by an equiproportional increase in the pollution control budget can lead to a change from stationary to cyclic behavior, especially where policies are based on resource abundance. 相似文献
56.
E. Warren Shows 《Journal of the American Water Resources Association》1983,19(2):249-253
ABSTRACT: Mathematical models have been widely used in business to minimize cost and maximize profit. In this paper such models are used to help determine the optimum strategy for a beach renourishment project including the initial and subsequent fill quantities and the length of a complete renourishment cycle. The importance of considering economic factors is emphasized, and the sensitivity of the optimal values to changes in the basic parameters of the models are discussed. The possible extensions and limitations of the models are considered. 相似文献
57.
ABSTRACT Sinkholes and subsidence in areas subject to dewatering of carbonate formations have been documented in several geologic and hydrologic investigations. Excessive ground-water pumpage has been cited as a causative factor in areas of increased sinkhole activity. Subsurface erosion of unconsolidated materials underlain by carbonate rocks is one cause of the collapses. Infiltrating water from retention basins may cause subsurface erosion and eventual failure by collapse of the structure. The resulting ground-water contamination would be great as the pollutants spread throughout the solution-cavity system. 相似文献
58.
G. Mathias Kondolf J. Warren Webb Michael J. Sale Thomas Felando 《Environmental management》1987,11(6):757-769
As the number of proposals to divert streamflow for power production has increased in recent years, interest has grown in predicting the impacts of flow reductions on riparian vegetation. Because the extent and density of riparian vegetation depend largely on local geomorphic and hydrologic setting, site-specific geomorphic and hydrologic information is needed. This article describes methods for collecting relevant hydrologic data, and reports the results of such studies on seven stream reaches proposed for hydroelectric development in the eastern Sierra Nevada, California, USA. The methods described are: (a) preparing geomorphic maps from aerial photographs, (b) using well level records to evaluate the influence of streamflow on the riparian water table, (c) taking synoptic flow measurements to identify gaining and losing reaches, and (d) analyzing flow records from an upstream-downstream pair of gages to document seasonal variations in downstream flow losses. In the eastern Sierra Nevada, the geomorphic influences on hydrology and riparian vegetation were pronounced. For example, in a large, U-shaped glacial valley, the width of the riparian strip was highly variable along the study reach and was related to geomorphic controls, whereas the study reaches on alluvial fan deposits had relatively uniform geomorphology and riparian strip width. Flow losses of 20% were typical over reaches on alluvial fans. In a mountain valley, however, one stream gained up to 275% from geomorphically controlled groundwater contributions. 相似文献
59.
Lance R. Williams Christopher M. Taylor Melvin L. Warren Jr. J. Alan Clingenpeel 《Environmental management》2002,29(1):76-87
Using Basin Area Stream Survey (BASS) data from the United States Forest Service, we evaluated how timber harvesting influenced
patterns of variation in physical stream features and regional fish and macroinvertebrate assemblages. Data were collected
for three years (1990–1992) from six hydrologically variable streams in the Ouachita Mountains, Arkansas, USA that were paired
by management regime within three drainage basins. Specifically, we used multivariate techniques to partition variability
in assemblage structure (taxonomic and trophic) that could be explained by timber harvesting, drainage basin differences,
year-to-year variability, and their shared variance components. Most of the variation in fish assemblages was explained by
drainage basin differences, and both basin and year-of-sampling influenced macroinvertebrate assemblages. All three factors
modeled, including interactions between drainage basins and timber harvesting, influenced variability in physical stream features.
Interactions between timber harvesting and drainage basins indicated that differences in physical stream features were important
in determining the effects of logging within a basin. The lack of a logging effect on the biota contradicts predictions for
these small, hydrologically variable streams. We believe this pattern is related to the large scale of this study and the
high levels of natural variability in the streams. Alternatively, there may be time-specific effects we were unable to detect
with our sampling design and analyses. 相似文献
60.
H. Sanei P.M. Outridge F. Goodarzi F. Wang D. Armstrong K. Warren L. Fishback 《Atmospheric environment (Oxford, England : 1994)》2010,44(13):1672-1681
This paper reports mercury (Hg) concentrations and fluxes in precipitation that was collected from 2006 to 2008 at three sites in Canada: sub-Arctic boreal forest, sub-Arctic coast, and southern Alberta, using cold-adapted precipitation collectors which operated reliably at temperatures below ?30 °C during the study. The southern Alberta site (Crossfield) may be influenced by Calgary urban air, whereas the sub-Arctic coastal (Churchill, Manitoba) and boreal forest (Fort Vermilion, Alberta) sites are in more remote northern areas. Annual mean Hg concentrations in precipitation (5.0–9.2 ng L?1) at the study sites were in the lower half of the range reported for southern Canada and the USA by the Mercury Deposition Network (MDN). But owing to typically low precipitation rates, gross wet Hg fluxes (0.54–2.0 μg m?2 yr?1) were among the lowest reported by MDN, with Crossfield having about twice the flux in 2007 of the other two sites. Flux was significantly correlated with precipitation, and thus was highest in summer (June–August) and lowest during winter, a pattern typical of other temperate continental locations. There was no evidence of higher wet Hg fluxes or concentrations in springtime at Churchill where atmospheric mercury depletion events (AMDEs) occur. Measured gross deposition fluxes at the study locations were ~2–8 times lower than estimated by GEOS-Chem and GRAHM atmospheric models. The largest discrepancy occurred for Churchill, which raises the question of how well Hg deposition from AMDEs is described by current models. Better agreement between measurements and models was obtained from MDN stations in Alberta and Alaska, where wet Hg fluxes were 2–10 times higher than the study sites either because of power plant emissions (Alberta), or because of high precipitation rates (Alaska). 相似文献